This is a project designed to characterize the maturation of lymphocyte precursors in the bone marrow of mice. We have found that maturing B cell precursors must go through 2 mitotic steps on their way to full maturity. The maturation of T cell precursors is more difficult to study, but we have shown that it is controlled by a thymic hormone which causes resting pre-T cells to enter cell cycle. We intend to characterize resting and proliferating pre-T cells by means of surface markers, the enzyme terminal deoxynucleotidyl transferase, and their ability to interact with antigen. We will also utilize our knowledge of the hormonal control of T cell maturation to establish, if possible, an in vitro cloning assay for thymic hormone, so that the hormone can be better characterized and purified, and the clonal restriction of pre-T cells examined.